REMOTE-CONTROLLED nano-devices that look like sperm but mimic the corkscrew motion of flagella may one day deliver drugs to where they are needed in the body.

Flagella are the structures some bacteria use to swim through water. Because water is syrupy at small scales, ordinary swimming motions don't work well. "Picture trying to swim in a pool of asphalt on a hot summer's day," says Peer Fischer of The Rowland Institute at Harvard University. Instead, flagella use a corkscrew motion to drive bacteria through the water.

The motion of flagella inspired Fischer and his colleague Ambarish Ghosh to create their nanopropellers. Made of glass, each has a spherical head 200 to 300 nanometres across and a corkscrew-shaped tail 1 to 2 micrometres long - less than one-tenth the length of a human sperm.

To make their propellers, Ghosh and Fischer covered a silicon wafer with glass beads, before depositing a vapour of silicon dioxide onto them. While doing so they spun the wafer, causing the silicon dioxide to form corkscrew-shaped tails on each bead. Finally, once the silicon dioxide had solidified they covered one side of the nanopropellers with cobalt.

Cobalt is magnetic, so when an external magnetic field is applied the propellers line up with the field. By making the field rotate, Ghosh and Fischer were able to make the propellers rotate with it, corkscrewing through the water at up to 40 micrometres per second (Nano Letters, DOI: 10.1021/nl900186w).

The nanopropellers can also be steered precisely. "We control the coils that give rise to the magnetic field," says Fischer. "By changing the magnetic field in three dimensions we can steer and propel the propellers." The team were able to get a single nanopropeller to trace out various characters, including an "R" and an "@".

Another benefit to using an external magnetic field to move the propellers is that the swimmers aren't limited by internal energy sources. It also means that the nanopropeller has no moving parts, unlike microbots.

Ghosh and Fischer have shown that a nanopropeller can push a silica bead over 1000 times larger than itself. Along with the propellers' size and controllability, that opens up a range of possible applications. Most exciting would be using them to deliver drugs to specific areas of the body via the bloodstream, or even to conduct surgery.

While a group at the Swiss Federal Institute of Technology in Zurich had previously produced corkscrew-shaped artificial swimmers, these new nanopropellers are more steerable and much smaller, says David Gracias, a nanobiotechnology researcher at Johns Hopkins University in Baltimore, Maryland. In fact, they are the smallest artificial swimmers yet. "It is an important step towards the creation of artificial mobile micro and nanoscale devices," says Gracias.